1. Field of the Invention
The present invention generally relates to a four-wheel drive vehicle that can transmit a portion of a drive force headed toward a main drive wheel to a subordinate drive wheel and distribute the portion of the drive force to a left subordinate drive wheel and a right subordinate drive wheel. More particularly, the present invention relates to a four-wheel drive vehicle drive force distribution control apparatus that controls a total drive force distributed to the left and right subordinate drive wheels and a drive force difference between the left and right subordinate drive wheels.
2. Background Information
An example of a conventional proposal for such a four-wheel drive vehicle and a drive force distribution control technology for such a vehicle is disclosed in Japanese Laid-Open Patent Publication No. 2007-185959 (See, FIGS. 1 and 5). The proposed technology has a total drive force control device and a left-right drive force difference control device to control a total drive force distributed to the left and right subordinate drive wheels and a drive force difference between the left and right subordinate drive wheels. In order to control the total drive force and the drive force difference, a target front-rear drive force distribution ratio between the main drive wheels and the subordinate drive wheels and a target left-right drive force distribution ratio between the left and right subordinate drive wheels are calculated. Then, the total drive force control device is operated such that the target front-rear drive force distribution ratio is achieved and the left-right drive force difference control device is operated such that the target left-right drive force distribution ratio is achieved.